1. Field of the Invention
This invention relates to a magneto-optical recording disc suitable for magneto-optical recording in a magnetic field modulation mode.
2. Prior Art
Optical discs are of great interest as high capacity information carrying media. Among the optical discs, magneto-optical discs of the magnetic field modulation system are expected to find use in data files and other advanced applications. A magneto-optical disc is recorded in a magnetic field modulation mode by directing a laser beam from an optical head to the recording layer of the disc in a DC manner for raising the temperature of the irradiated spot, and applying a modulated magnetic field across the recording layer from a magnetic head opposed to the optical head at the same time. Therefore, the magnetic field modulation mode allows for overwrite recording.
Most of conventional magneto-optical disc drives depend on a contact start and stop (CSS) system using a flying magnetic head. The magnetic head is in contact with the disc surface at the start and end of disc rotation. Thus the disc on the magnetic head facing surface is provided with a protective coating for protecting the magnetic head from adsorption and crushing.
For the protective coatings on magneto-optical discs of the CSS system, various resin compositions were proposed. For example, Japanese Patent Application Kokai (JP-A) Nos. 301040/1990 and 301041/1990 disclose a resin composition predominantly comprising a fluorinated polyurethane resin and JP-A 37844/1991 discloses a resin composition predominantly comprising a fluoro-resin soluble in organic solvents. It is also proposed to incorporate metal elements and lubricants in these resin compositions, the metal elements having greater ionization tendency than the transition metal element of which the recording film is made. JP-A 98857/1988 discloses to incorporate fluorinated carbon lubricant into resins such as epoxy resins. JP-A 40149/1990 discloses UV-curable resins having blended therein lubricants such as higher fatty acid esters (e.g., stearates), higher fatty acids, higher fatty acid amides, and silicone oil, JP-A 17844/1991 discloses a lubricating layer, and JP-A 62338/1991 discloses to disperse fine particulates of silicone resin or nickel on an overcoat layer for uniformly distributing asperities of 0.1 to 0.5 .mu.m high on the surface. JP-A 222349/1988 and 70944/1989 provide an anti-crushing film on a protective film on a magneto-optical recording layer, the anti-crushing film on its surface being provided with a plurality of discrete micropores where the film is impregnated with liquid lubricant. Furthermore, JP-A 64937/1992 discloses a protective layer of a curable resin containing a plurality of voids where the layer is impregnated with lubricant.
Magneto-optical discs capable of recording/reading operation at the same normal chain velocity as compact discs (CD) are recently highlighted since they can share the drive unit with the CDs simply by adding or modifying the optical system. The CDs employ a normal chain velocity as low as 1.2 to 1.4 m/sec. at which the magnetic head cannot be aerodynamically lifted. Then the CSS system cannot be resorted to. One would attempt to use a stationary magnetic head which is held at a fixed, close distance from the disc surface. The stationary magnetic head, however, can collide with the disc due to axial runouts of the disc and vibration of the drive unit. Servo means must be provided in order to allow the head to follow axial runouts of the disc, resulting in a complex drive mechanism.
Under the circumstances, we reached the concept of a magnetic head in continuous contact with the disc surface and made research works on a magneto-optical disc capable of such operation. It was found that when the magnetic head was in sliding contact with the disc surface, the protective coatings previously proposed for the CSS system were unsatisfactory.
The protective coating having lubricant incorporated therein fails to retain lubricity for a sufficient time since the magnetic head is in continuous sliding contact with the protective coating surface. More specifically, the fluorinated carbon lubricants disclosed in JP-A 98857/1988 and the conventional stearates among higher fatty acid esters, higher fatty acids or ester amides thereof and silicone oil disclosed in JP-A 40149/1990 suffer from many problems including an increase in coefficient of friction during storage or usage, typically storage at high temperature, noise generation due to head sliding, a loss of dynamic performance, jitter occurrence, and low durability. Attempts to disperse fine particles in a protective coating in order to increase the surface roughness have the problems of noise generation due to head sliding and jitter occurrence. Fine particles of metal or ceramics can cause damage to the magnetic head because of their hardness. Protective coatings with micropores or voids where the coatings are impregnated with lubricant require a special resin material and a complex manufacturing step and have a short effective life because of insufficient durability and friction properties.
Among the above-mentioned protective coatings, a protective coating having a lubricant incorporated in a UV-cured resin is obtained by adding the lubricant to a UV-curable compound and applying the composition to form a coating, followed by curing. If the UV-curable compound and the lubricant are less compatible, the composition could not be effectively coated or the resulting coating would have unsatisfactory properties and insufficient hardness. Then this protective coating is less effective for reducing friction and fails to function as a wear resistant coating.